Memory-efficient ADSL transmission in the presence of TCM-ISDN interferers

ABSTRACT

A method of communicating data across a channel that experiences near-end cross talk (NEXT) interference and far-end cross talk (FEXT) interference in alternate intervals. In one embodiment, a first data rate is determined for a first carrier-number mode that is to utilize a first bit table, a second data rate is determined for a second carrier-number mode that is to utilize dual bit tables, a third data rate is determined for a third carrier-number mode that is to utilize a second bit table during a FEXT interval, and a modem is configured to transmit using the mode having a highest data rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. Section 120 to U.S.patent application Ser. No. 10/342,059 filed Jan. 14, 2003. Saidapplication herein incorporated by reference.

BACKGROUND

Telephone companies offer customers a number of ways to transport data.One popular way is called Asymmetric Digital Subscriber Line (ADSL). InADSL, a small portion of the frequency spectrum is used forcommunicating data from the customer to the central office, and a muchlarger portion of the frequency spectrum is used for communicating datafrom the central office to the customer.

Discrete multi-tone (DMT) modulation is used within each portion of thefrequency spectrum, i.e., data are carried on equally-spaced carriersignals. The combined number of carrier signals from both portions ofthe spectrum is implementation-dependent. ADSL implementations thatcomply with the ITU-T G.992.1 standard have 256 carrier signals, whileimplementations that comply with the ITU-T G.992.2 standard have 128carrier signals. Future implementations are expected to have 512 or even1024 carrier signals (see, e.g., ITU-T G.992.5).

DMT modulation provides for very efficient use of the availablecommunication spectrum because the amount of data carried by eachcarrier signal is individually customized to fit the signal-to-noiseratio profile of the channel. Each carrier signal is allocated a numberof data bits, and the allocation of bits may be dynamically adjusted aschannel conditions change. Each carrier signal may also be allocated asmall individual gain factor to further improve communicationsperformance.

The allocation of bits and gain factors to carrier signals are typicallyperformed using tables. A gain table includes an individual gain factorfor each carrier signal. A bit table includes an individual number ofbits allocated to each carrier signal. A tone table may be used toallocate specific data bits to specific carrier signals.

Some channels actually have two signal-to-noise ratio profiles. Anexample of such a channel is a twisted wire pair in a binder that alsocarries TCM-ISDN (Time Compression Multiplexing-Integrated ServicesDigital Network) traffic. TCM-ISDN employs time division multiplexing ata rate of 400 Hz, i.e. the central office alternately transmits data for1.25 milliseconds, then listens for data from the customer for 1.25milliseconds. This causes other channels to experience a noise profilethat alternates at a rate of 400 Hz.

The interference can be divided into two types: near-end cross talk(NEXT) and far-end cross talk (FEXT). Some confusion can arise whendiscussing NEXT and FEXT since the meaning of NEXT and FEXT changes withrespect to the chosen reference point. For clarity herein, the centraloffice is hereby chosen as the arbitrary reference point, and thisreference point will be used consistently throughout. NEXT interferenceon a given channel is caused by central office transmissions on otherchannels. FEXT interference on a given channel is caused bytransmissions from customers on other channels. TCM-ISDN signalingalternately causes NEXT interference and FEXT interference. The NEXTinterference is generally significantly worse than the FEXTinterference, although this depends on the distance that the twistedwire pair travels alongside interfering channels.

The ITU-T G.992.1 and G.992.2 standards each address TCM-ISDNinterference in their respective Annex C. Two solutions are offered:dual mode solution and FEXT-only solution. In the dual mode solution,two sets of tables (gain, bit, and tone) are used. One set of tables isused to construct symbols for transmission during periods of NEXTinterference (“NEXT symbols”), and the other set of tables is used toconstruct symbols for transmission during periods of FEXT interference(“FEXT symbols”). Although TCM-ISDN signaling uses a 50% duty cycle, itis expected that on average, only 126/340 (about 37%) of the symbolswill be free of NEXT interference, and hence constructible as FEXTsymbols.

Although the dual mode solution offers a higher data rate, it does addsignificant cost to the modem in the form of additional memory for thesecond set of tables. This additional cost is expected to be significantfor future ADSL implementations having 512 or more carrier signals dueto the increased size of the tables.

The FEXT-only solution is similar to the dual mode solution except thatno symbols are constructed or sent during the periods of NEXTinterference. Because only FEXT symbols are used, only one set of tablesis needed. Although the FEXT symbols typically carry more data than NEXTsymbols, sacrificing 63% of the symbols can impose a substantialperformance penalty.

Accordingly, it would be desirable to have a memory-efficient method forADSL transmission in a time-varying noise environment. Such a methodwould preferably avoid the performance penalty of the FEXT-only solutionwithout suffering the prohibitive expense of the dual mode solution.

SUMMARY

Accordingly, there is disclosed herein a method of communicating dataacross a channel that experiences near-end cross talk (NEXT)interference and far-end cross talk (FEXT) interference in alternateintervals. In one embodiment, the method comprises: a) determiningN_(F), the number of bits per symbol usable in a FEXT-only mode ofoperation; b) determining N_(S), a number of bits per symbol usable in asingle mode of operation; c) determining whether the FEXT-only mode orthe single mode provides a higher data rate; and d) configuring a modemto transmit using the mode having a higher data rate. The FEXT-only modemay be determined to have a higher data rate when 126N_(F)>340N_(S).

In another embodiment, the method comprises: a) performing a bit tabledetermination procedure for FEXT symbols; b) performing a bit tabledetermination procedure for NEXT symbols; and c) constructing a bittable for single mode symbols. The single mode bit table may beconstructed by setting each single mode bit table entry equal to thelesser of the corresponding entries in a FEXT symbol bit table and aNEXT symbol bit table. The number of bits per symbol usable in theFEXT-only mode may be determined by summing entries from the FEXT-onlysymbol bit table, and the number of bits usable in the single mode maybe determined by summing entries from the single mode symbol bit table.

In yet another embodiment, the method comprises: a first modemdetermining that FEXT-only mode or single mode is preferredcommunication mode; a second modem determining that FEXT-only mode orsingle mode is a preferred communication mode; and each modemtransmitting using the preferred communication mode of the other modem.The preferred communication modes may be different.

Also contemplated is a modem that comprises: a memory and a processor.The memory is configured to store a single bit table. The processor isconfigured to transmit and receive data via a channel that experiencesalternate intervals of NEXT and FEXT interference. The processordetermines the number of bits per symbol usable in FEXT-only mode andsingle mode, and stores in memory the bit table for the mode offeringthe higher data rate.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredin conjunction with the following drawings, in which:

FIG. 1 is a functional-block diagram of one ADSL modem embodiment;

FIG. 2 is a schematic block diagram of the ADSL modem embodiment of FIG.1;

FIG. 3A is a flow diagram of a setup method of a preferred ADSL modemembodiment;

FIG. 3B is a flow diagram of a setup method of a preferred ADSL/ADSL+modem embodiment;

FIG. 4 is a graph showing relative data rates of different downlinkcommunication modes on a time-varying noise channel; and

FIG. 5 is a graph showing relative data rates of different uplinkcommunication modes on the time-varying noise channel.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION

Turning now to the figures, you will find a functional block diagram ofone embodiment of an ADSL (“Asymmetric Digital Subscriber Line”) modem100 in FIG. 1. A control and data bus conveys to modem 100 data that isto be transmitted. A multiplexer/synchronization controller 102 routesthe data via two paths: a fast path, and an interleaved path. Data thatneeds to be sent with a low latency is routed through the fast path,while data that needs to be sent with a low error rate is routed throughthe interleaved path. The two data types may be intermixed andidentified by a bus protocol.

The fast path data is delivered to CRC (cyclic redundancy code) block104, which aligns the data into frames and appends a CRC checksum value.The interleaved path data is delivered to an independent CRC block 106for the same processing. CRC blocks 104 and 106 forward the data framesto scrambling and forward error correction (FEC) blocks 108 and 110,respectively.

Blocks 108 and 110 each combine on a bit-wise basis the bits of theframe data with a pseudo-random bit sequence to randomize, or“scramble”, the data. An error correction coding process is then appliedto the randomized frame data to add redundancy. This redundancy may beused by the receiver to detect and correct errors caused by channelinterference. In the fast path, block 108 provides the randomized andencoded data to rate converter block 112, while in the interleaved path,block 110 provides the randomized and encoded data to an interleaverblock 114.

Interleaver block 114 re-orders the data in the stream of randomized andencoded data frames in a manner that interleaves the data from one frameamong the data from other frames. By “spreading out” the data of a givenframe in this manner, the modem provides increased resistance to “errorbursts” on the channel. That is, a closely-spaced set of errors causedby channel interference will not be concentrated in the data of a singleframe, but rather, the errors will be “scattered” at the receiver sothat only one or two errors will appear in the data of a given frame.This prevents an error burst from overwhelming the error correctionability of the error correction code, however, it also introduces asignificant latency in the travel time of the data. The stream ofinterleaved data is provided to rate converter block 116.

Rate converter blocks 112 and 116 may be buffers designed to “even out”the data rate required from the preceding blocks. The following blocksoperate at two different data rates because the modem uses differentdata rates during periods of NEXT and FEXT interference.

A tone ordering block 118 retrieves data streams from the rate converterblocks 112, 116, and allocates bits from the data streams to outgoingcarrier signals in accordance with a tone ordering table. It isgenerally preferred to allocate the bits in a manner that avoids havingadjacent bits of the data stream carried on the same or adjacent carriersignals. Spreading out the bits in the frequency domain provides addedresistance to interference that is localized in a given frequencyregion.

A constellation encoding and gain scaling block 120 receives thecarrier-allocated bits from block 118 and converts them into amplitudevalues for the carriers. The conversion may be done in two steps: Firstthe bits may be mapped to a quadrature amplitude modulation (QAM)constellation point, e.g. a four-bit value corresponds to apredetermined point in a 16-point QAM constellation. Second, theconstellation point coordinates are multiplied by a gain value for thecarrier signal to obtain the carrier signal amplitude.

Note that blocks 118 and 120 may rely on one or two sets of toneordering, bit, and gain tables. A modem operating in the dual mode mayemploy two sets of tables, while a modem operating in a FEXT-only modemay employ just one set of tables.

An inverse Discrete Fourier Transform (IDFT) block 122 applies aninverse Discrete Fourier Transform to the carrier signal amplitudes toobtain one “symbol”, that is, samples of a time domain signal having afrequency spectrum with the specified carrier signal amplitudes. Block124 converts the symbol to serial form. The serial form of the symbolincludes a cyclic prefix. The cyclic prefix is a copy of some number ofsamples from the end of the symbol, added as a prefix to the symbol.This prefix causes the convolution with the channel impulse response tomimic cyclical convolution, allowing for simpler equalization at thereceiver.

Block 126 converts the sequence of symbols in serial form into an analogsignal, which may then be filtered and amplified to form a transmitsignal. The transmit signal is supplied via hybrid 128 to the channel,through which it travels to a receiver. A transmitter at the other endof the channel simultaneously transmits a signal for reception by modem100.

The hybrid 128 operates to extract the receive signal from the channelwhile at the same time conveying a transmit signal to the channel. Thehybrid may include one or more bandpass filters to prevent the transmitsignal from interfering with the receive signal.

The receive signal from hybrid 128 may be equalized by block 130 andconverted into digital form. Block 132 converts the sequence of symbolsin serial form into symbols in parallel form, dropping the cyclicprefix. Block 134 applies a Discrete Fourier Transform (DFT) to thesymbols, thereby determining amplitudes of carriers in the symbol.However, the amplitudes have been affected by transit through thechannel.

Block 136 applies a spectral filter to the carrier amplitudes. Aspectral filter is a gain factor for each carrier that compensates forchannel attenuation and transmit gain table. Block 136 also maps thefiltered carrier amplitudes to bits represented by the closestconstellation points.

The detected bits are re-ordered by block 138 to put adjacent bits backtogether. Block 138 further segregates the bits into two bit streams,one for the fast path, and one for the interleaved path. The bit streamsare provided to respective rate converter blocks 140, 142 to smooth thedata rate for the ensuing blocks.

Note that blocks 136 and 138 may rely on one or two sets of filter, bit,and tone ordering tables to perform their functions. Two sets would beused when operating in dual mode, whereas only a single set would beneeded when operating in FEXT-only mode.

The rate converter block 140 for the fast path provides a sequence ofrandomized and encoded data frames to block 144, while the rateconverter block 142 for the interleaved path provides an interleaveddata stream to block 146. Block 146 performs a de-interleaving processon the interleaved data stream to reconstruct a sequence of randomizedand encoded data frames. The sequence is provided to block 148.

Blocks 144 and 148 perform an error correction (“decoding”) process anda de-scrambling process on the randomized and encoded data frames. Thedecoding process identifies and corrects channel-induced errors in thedata, dropping the previously-introduced redundancy in the process. Thede-scrambling process again applies the pseudo-random bit sequence in abit-wise fashion to undo the data randomization. Blocks 144 and 148provide data frames to blocks 150 and 152, respectively.

Blocks 150, 152 perform a CRC checksum confirmation. If the CRC checkfails, some form of error handling is carried out—typically a processorinterrupt may be asserted by modem 100, thereby causing the processor torequest that the data be re-transmitted. Assuming that the CRC checksucceeds, the data is extracted from the frame structure and provided toblock 154. Block 154 makes the data streams from the fast andinterleaved data paths available to the system via the control and databus.

Further details on the operation of the above-described embodiment ofmodem 100 are available in the ITU-T G.992.1 (06/99) standard, which ishereby incorporated by reference. Special reference should be made toAnnex C of the standard.

FIG. 2 shows a preferred implementation of modem 100, in which many ofthe functional blocks are implemented by a digital signal processor(DSP) 202 that operates in accordance with software 203 stored in amemory 204. Memory 204 may also include one or two sets of tables, eachset consisting of a gain table 206, a bit table 208, a tone table 210,and a filter table 212. The DSP 202 may be coupled to a system bus viainterface logic 214.

The above-described embodiment of modem 100 operates in one of twomodes: dual mode or FEXT-only mode. The operating mode may be determinedby the capabilities of the modem on the other end of the channel. If theother modem supports dual mode, then dual mode will be used. OtherwiseFEXT-only mode is used. The dual mode may support over twice the datarate of the FEXT-only mode on short loops. However, the increased costto support the dual mode may be prohibitive due to the memory requiredfor a second set of tables.

In a first preferred, alternative embodiment, the modem 100 may operatein one of two modes: single mode or FEXT-only mode. In the single mode,a single set of tables is used for constructing and decoding symbolssent during both NEXT- and FEXT-interference periods (“NEXT symbols” and“FEXT symbols”, respectively). In the FEXT-only mode, a single set oftables is used for constructing and decoding symbols send during theFEXT-interference periods, and no symbols are sent during the NEXTinterference periods. It is expected that this first preferredembodiment will nearly achieve the dual mode data rates at the samecomplexity as a modem that supports only the FEXT-only mode. Theadvantages of this embodiment will become more pronounced as futuretechnologies support a greater number of carrier signals and requirelarger tables.

In a second preferred embodiment, the modem 100 may support differentnumbers of carrier signals (e.g. 256 in ADSL, and 512 in ADSL+). Whenoperating at the higher number of carrier signals, the modem may supportonly those modes requiring a single set of tables, i.e. single mode andFEXT-only mode. However, when operating at the lower number of carriersignals, the modem in the second preferred embodiment also supportsmodes requiring two sets of tables, i.e. the dual mode. The addedflexibility of this embodiment is expected to enhance performancerelative to the first preferred embodiment.

In the preferred embodiments, modem 100 will determine modes supportedby the modem on the other end of the channel, and will avoid operatingin modes not supported by the other modem. Hence, if the modem on theother end of the channel does not support single mode, modem 100 willoperate in FEXT-only mode or dual mode (if supported). Assuming allmodes are supported by the other modem, the software 203 causes the DSP202 in the preferred modem embodiments to follow the appropriate setupprocedure shown in FIG. 3A or 3B.

In block 302 of FIG. 3A, the modem characterizes the channel,identifying the signal-to-noise ratio profiles for bothNEXT-interference periods and FEXT-interference periods. In block 304,the modem goes through the bit table (and possibly the gain table)determination procedure assuming operation in the FEXT-only mode. Atleast one result of block 304 is a calculation of N_(F), the number ofbits that would be carried by each FEXT symbol.

In block 306, the modem calculates an expected data rate for operationin the FEXT-only mode. The expected data rate may be determined exactlyor an approximation may be used, since the exact data rate calculationmay be too involved. The expected FEXT-only data rate (R_(FO)) may becalculated in accordance with the following expression:

$\begin{matrix}{{R_{FO} = {\left( \frac{N_{F}\mspace{14mu}{bits}}{{data}\mspace{14mu}{symbol}} \right) \cdot \left( \frac{126}{340} \right) \cdot \left( \frac{4000\mspace{14mu}{data}\mspace{14mu}{symbols}}{second} \right)}},} & (1)\end{matrix}$where 126/340 is the overall fraction of symbols that are free fromNEXT-interference.

In block 308, the modem goes through the bit table (and possibly gaintable) determination procedure assuming operation in the single mode.The bit table is determined from a combination of the bit tables for theNEXT and FEXT symbols, although it is expected to be unnecessary tocompletely determine the bit tables separately. The FEXT symbol bittable was determined in block 304. As the bit table for the NEXT symbolsis determined, it is combined with the bit table for the FEXT symbols todetermine the single mode bit table.

Recall that the bit table specifies the number of bits allocated to eachcarrier signal. The single mode bit table has for each carrier thelesser of the two numbers in the bit tables for the FEXT and NEXTsymbols. Thus the number of bits for a given carrier in a NEXT symbol iscompared to the number of bits for a given carrier in a FEXT symbol, andthe smaller number is stored in the bit table for the single mode. Atleast one result of block 308 is a calculation of the N_(S), number ofbits that would be carried by each symbol.

If gain table calculations are also being performed at this stage, thegain factor for a given carrier is in accordance with the selected mode(i.e., FEXT-only mode or single mode). That is, if FEXT-only mode ischosen, the gain table is preferably computed for the bit table used inFEXT-only mode. If single mode is chosen, the gain table is computed forthe bit table used in single mode.

In block 310, the modem calculates an expected data rate for operationin the single mode. As before, the expected data rate may be determinedexactly or an approximation may be used. The expected single mode datarate (R_(SM)) may be calculated in accordance with the followingexpression:

$\begin{matrix}{R_{SM} = {\left( \frac{N_{S}\mspace{14mu}{bits}}{{data}\mspace{14mu}{symbol}} \right) \cdot {\left( \frac{4000\mspace{14mu}{data}\mspace{14mu}{symbols}}{second} \right).}}} & (2)\end{matrix}$

In block 312, the modem compares the data rates for the two modes. In analternative embodiment, blocks 306 and 310 may be omitted, and theequations (1) and (2) may be collapsed into a single comparison todetermine whether:126N_(F)>340N_(S).  (3)

The modem makes a decision based on the outcome of the comparison. Ifthe data rate for the FEXT-only mode is greater, then in block 314, themodem configures for FEXT-only operation. If the FEXT-only bit table hasbeen overwritten, the modem may repeat the FEXT-only bit tabledetermination. If the data rate for the single mode is greater, then inblock 316, the modem configures for single mode operation.

Note that the operating mode is preferably determined by the receivingmodem, so it is contemplated that modems at both ends of the channelperform this procedure. Note that the outcome may be different for eachmodem, so it is conceivable that one modem may be transmitting using theFEXT-only mode and receiving using the single mode.

The setup procedure shown in FIG. 3B is similar to that of FIG. 3A.However, in FIG. 3B, the procedure is extended to consider operationwith different numbers of carriers. Such a circumstance may arise in amodem that supports both ADSL (256 carriers) and ADSL+ (512 carriers).In this procedure, the data rates for high carrier number FEXT-only mode(“FO”); high carrier number single mode (SM); and low carrier numberdual mode (“DM_(LO)”) are determined and compared. Thus in block 304,the FO bit allocation is determined and the number of bits per symbolN_(F) is calculated. The data rate calculation of block 306 isunchanged. In block 308, the SM bit allocation is determined and thenumber of bits per symbol N_(S) is calculated. Again the data ratecalculation of block 310 is unchanged.

In block 320, the dual bit table allocation procedure is followed andthe number of bits for FEXT symbols (N_(F-LO)) and the number of bitsfor NEXT symbols (N_(N-LO)) are determined. The data rate calculation inblock 322 may take the following form:

$\begin{matrix}{{R_{{DM}\text{-}{LO}} = {\left\lbrack {{\left( \frac{N_{F\text{-}{LO}}\mspace{14mu}{bits}}{{data}\mspace{14mu}{symbols}} \right) \cdot \left( \frac{126}{340} \right)} + {\left( \frac{N_{N\text{-}{LO}}\mspace{14mu}{bits}}{{data}\mspace{14mu}{symbol}} \right) \cdot \left( \frac{214}{340} \right)}} \right\rbrack \cdot \left( \frac{4000\mspace{14mu}{data}\mspace{14mu}{symbols}}{second} \right)}},} & (4)\end{matrix}$

In block 324, the modem compares the data rates for the three modes. Inan alternative embodiment, blocks 306, 310 and 322 may be omitted, andthe equations (1) and (2) (as calculated for the high carrier number)may be collapsed with equation (4) into three-way comparison todetermine which of the following is largest:max{126N_(F),340N_(S),(126N_(F-LO)+214N_(N-LO))}  (5)

The modem makes a decision based on the outcome of the comparison. Ifthe data rate for the FO+ mode is greater, then in block 314, the modemconfigures for FO operation. If the FO bit table has been overwritten,the modem may repeat the FO bit table determination. If the data ratefor the SM mode is greater, then in block 316, the modem configures forSM operation. Again, if the SM bit table has been overwritten, the modemmay repeat the SM bit table determination. Finally, if the DM_(LO) modehas the largest data rate, then in block 326, the modem configures forDM_(LO) operation.

FIGS. 4 and 5 compare performance simulations for the single mode, dualmode, and FEXT-only mode. These simulations were done considering only asingle number of carriers, namely, 256 (ADSL). FIG. 4 shows theperformance of a modem transmitting from the central office(downstream), and FIG. 5 shows the performance of a modem transmittingfrom the customer (upstream). The following assumptions were used forthe simulations:

Downstream band carriers 36-255 Dwnstream Transmit PSD Mask −40 dBm/Hzconstant Upstream band carriers 6-31 Upstream Transmit PSD Mask −38dBm/Hz constant Coding gain 3 dB Margin 6 dB Allowed range ofbits/carrier 1-15The assumed noise environment assumed 9 TCM-ISDN FEXT and NEXTinterferers in the same binder, and 9 ADSL FEXT disturbers (no NEXTdisturbers due to frequency division duplex operation). The downstreamreceiver experienced −140 dBm/Hz of additive white Gaussian noise, andthe upstream receiver experienced −123 dBm/Hz of additive white Gaussiannoise.

The NEXT- and FEXT-interference models used for the simulations are asfollows:

$\begin{matrix}{{PSD}_{FEXT} = {{PSD}_{DISTURBER} \times {{H(f)}}^{2} \times 10^{- \frac{X_{FEXT}}{10}} \times \left( \frac{l}{l_{0}} \right) \times \left( \frac{f}{f_{0}} \right)^{2}}} & (6) \\{{PSD}_{NEXT} = {{PSD}_{DISTURBER} \times 10^{- \frac{X_{NEXT}}{10}} \times \left( \frac{f}{f_{0}} \right)^{1.5}}} & (7)\end{matrix}$where f₀=1 MHz, l₀=500 m, and X_(FEXT) and X_(NEXT) are the crosstalkcoefficients. Cables using color-coded polyethylene (CCP) coatings haveX_(FEXT)=36.6 dB and X_(NEXT)=40.2 dB. Cables using paper coatings(performance not shown in figures) have X_(FEXT)=28.6 dB andX_(NEXT)=38.0 dB. The figures assume 0.4 mm conductors.

FIGS. 4 and 5 each show 3 curves. The solid curve shows the performanceof a modem using dual mode operation, the dash-dotted curve shows theperformance of a modem using FEXT-only mode, and the dashed curve showsthe performance of a modem using single mode. In the single mode case,the bit loading for all symbols is performed based on the worst casenoise which is usually NEXT-interference for all loop lengths.

The FEXT-only mode demonstrates relatively poor performance for shortloop lengths, but converges to the dual mode level of performance atlong loop lengths. Conversely, the single mode demonstrates relativelypoor performance for long loop lengths, but converges to the dual modeperformance for short loop lengths. Hence the preferred modem embodimentperforms well at both extremes and suffers only minor degradationrelative to the dual mode at intermediate lengths. Since the crossoverpoint is different for upstream and downstream directions, the preferredembodiment further enhances performance by allowing for the use ofdifferent modes in upstream and downstream communications.

It is noted that the constants 126, 214 and 340 were chosen asappropriate for the preferred embodiments, but other numbers may provesuitable for different embodiments.

Numerous variations and modifications will become apparent to thoseskilled in the art once the above disclosure is fully appreciated. It isintended that the following claims be interpreted to embrace all suchvariations and modifications.

1. A method of communicating data across a channel that experiencesnear-end cross talk (NEXT) interference and far-end cross talk (FEXT)interference in alternate intervals, the method comprising: determininga first data rate for a first carrier-number mode that is to utilize afirst bit table; determining a second data rate for a secondcarrier-number mode that is to utilize dual bit tables; determining athird data rate for a third carrier-number mode that is to utilize asecond bit table during a FEXT interval, wherein the first and thirdcarrier-number modes comprise a same number of carriers, the first andsecond carrier-number modes comprise a different number of carriers, andthe first single bit table is to be used during a FEXT interferenceinterval and a NEXT interference interval; and configuring a modem totransmit using the mode having a highest data rate.
 2. The method ofclaim 1, wherein the second carrier-number mode comprises an ADSL modeof operation, and the first carrier-number mode comprises an ADSL+ modeof operation.
 3. The method of claim 1, wherein a first one of the dualbit tables is to be used for a FEXT interference interval, and a secondone of the dual bit tables is to be used for a NEXT interferenceinterval.
 4. The method of claim 1, further comprising: sending nosymbol during a NEXT interval; and sending a symbol during a FEXTinterference interval based on the first bit table.